DBCC CHECKIDENT with RESEED lets you reset table identity to any value. Reseed to 0 so the next insert gets 1, and never go below values already in the table.
SQL SERVER – Primary Key Constraints and Unique Key Constraints
Primary Key and Unique Key both enforce uniqueness, but they differ in the index they create and in NULL handling. Here are the differences and the scripts.
SQL SERVER – ReIndexing Database Tables and Statistics with Targeted Maintenance
ReIndexing Database tables starts with a measured maintenance need. I check the workload before choosing an index operation.